With Valentine’s Day right around the corner there are hearts to be found everywhere— in shop windows, covering cards at the local pharmacy and in paper cut-outs made in school. Those stylized shapes are what we think of when someone talks about a heart, but have you ever wondered how close that is to a real, beating heart? Or how that heart manages to ship gallons and gallons of blood around the body 24/7?
The heart is basically a pump. Its job is to receive oxygen-poor blood from the body, send it to the lungs to get oxygen and then back out into the body. In dogs and cats (and people), the heart has four spaces, or chambers, to hold blood. Blood enters the heart from the vena cava— the largest vein in the body. Veins are vessels that take blood to the heart. It enters the first chamber, called the right atrium. When the heart beats, the blood is pushed into the chamber below, called the right ventricle. The next beat pushes the blood out through the pulmonary artery into the lungs. Arteries are vessels that carry blood away from the heart. The lungs provide oxygen which is carried by the red blood cells and then the blood is delivered back to the heart via the pulmonary veins. An interesting fact is that the pulmonary artery is the only artery in the body that carries de-oxygenated blood and the pulmonary veins are the only ones which carry oxygen-rich blood. The pulmonary veins discharge the blood into the left atrium and the next beat sends it into the left ventricle. From there, it is jettisoned into the aorta, the largest artery in the body, and goes to the rest of the body. The muscle making up the left ventricle is thicker than the one making up the right because it needs to be able to send blood throughout the whole body while the right only has to send it into the lungs. This gives the heart an asymmetrical shape, unlike those perfect candy hearts.
One-way valves are present throughout the heart to prevent the blood from going back the way it came when the heart beats. There is a valve in the pulmonary artery and the aorta to keep blood from flowing back into the heart. The valve between the right atrium and right ventricle is called the tricuspid valve. It has three pieces, or cusps, hence the name. The mitral valve divides the left and atrium and ventricle and has two cusps. Its name comes from its resemblance to a bishop’s hat, called a mitre. When you hear the “lub-dup” sound of a heartbeat, you are actually hearing the echo of the valves slamming shut.
The actual beating of the heart is controlled by an electrical system and is what we look at when we read an EKG. There is a bundle of nerves in the heart called the sinoatrial node (or SA node). This small region is the heart’s pacemaker. The beat is initiated in the SA node and causes the atria to contract, pushing blood into the ventricles. That nerve impulse travels downwards to another node called the atrioventricular (or AV) node. The impulse slows just enough to give the atria time to finish their contraction before the ventricles contract. When the ventricles contract, the mitral and tricuspid valves close and the blood circulates through the heart in one direction.
The contraction phase of the heartbeat is called systole. When you have your blood pressure measured, that is the top number and is the higher one because that’s when the pressure in the vessels is the highest. When the heart rests in between contractions, that gives time for the chambers to fill again with blood. That phase is called diastole and is the lower number.
There are many things that can go wrong with the heart, but unlike people, who are prone to getting heart attacks (technically called myocardial infarctions), dogs and cats get other types of heart diseases.
So next time you bring your pet in for an exam and we listen to his heart, think about all that is going on in there. And if you’d like to listen, just ask. (You don’t have to be a kid— it’s cool for adults, too).